The Lake Oroville dam stress test isn’t over — more rain this week, then spring thaw

More rain is expected in Northern California and the melting season is just around the corner. Here's what all that extra water means for the Lake Oroville dam. (Claritza Jimenez,Angela Fritz/The Washington Post)

Nearly 200,000 evacuees may soon be allowed to return to their homes as the threat of a dam breach diminishes at Lake Oroville. But while the immediate risk is subsiding, the long-term outlook is less promising, or — at the least — more uncertain. Three storms are lined up to drench Northern California over the next week. Beyond that, the melting season looms with more snow piled on the peaks of the Sierra than there has been in years.

After Sunday night’s unprecedented evacuation downstream from the Oroville dam, a flash flood warning was still in effect Monday afternoon. Of course it has very little to do with current weather. Should a torrent of water come rushing through valleys and across roads, it will be caused by the failure of a man-made spillway system that has never been tested and has a questionable history of safety oversight.

The reservoir level is falling at three to four inches per hour, the L.A. Times reports, and was four feet below the emergency spillway Monday afternoon. But the Department of Water Resources wants to reduce the level to 50 feet below the emergency spillway, which has workers “scrambling” given the forecast.

Up to nine inches of precipitation is possible in the region over the next seven days. That’s what global weather forecast models are predicting as three storms line up back-to-back for Northern California. After a relatively dry Monday and Tuesday, rain will return Wednesday and last through early next week.

Even if we assume widespread rainfall totals of six inches in the Oroville runoff area, that would mean 400 billion gallons, or 1.23 million acre-feet, of water input over the next five to 10 days.

Precipitation forecast through Sunday evening from the GFS model. (pivotalweather.com)

“River levels will rise and continue to pose problems late this week and into early next week due to more weather systems, but lower snow levels may limit flooding impacts,” the National Weather Service said in a Monday briefing.

That’s good news for the rest of the region, but the spillway failure will be affected by any amount of additional rainfall and snowmelt between now and the time it’s repaired, which may be on the order of months. In the meantime, the exceptional rainfall year continues.

Though Feb. 10, not a single climate-monitoring station in Northern California is reporting below-average rainfall since the middle of 2016. At least three stations — Sacramento, Blue Canyon and Santa Rosa — have at least doubled the amount of precipitation they usually receive since July 1.

Authorities ordered an emergency evacuation in Oroville, Calif., after a damaged spillway threatened the area with flooding. Here's what you need to know about the situation. (Video: Monica Akhtar / The Washington Post; Photo: Stephen Lam for The Washington Post)

In fact, by one important measure, there’s been more rain and snowfall in the 2016-2017 water year than any other season on record, to date. The California-Nevada River Forecast Center uses an eight-station index in the North Sierra to quantify the region’s precipitation. As of Feb. 12, these eight stations have received 68 inches — 226 percent of normal.

Geographically, the swath that has received the majority of rain and snowfall is about 60 miles wide with Interstate 80 along the southern edge. It happens to be exactly the region that drains to Lake Oroville.

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See photos as thousands are evacuated after Oroville Dam threatens floods

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About 188,000 residents near Oroville, Calif., were ordered to evacuate after a hole in an emergency spillway in the dam threatened to flood the surrounding area.

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About 188,000 residents near Oroville, Calif., were ordered to evacuate after a hole in an emergency spillway in the dam threatened to flood the surrounding area.

Feb. 17, 2017 This photo shows water continuing to move down the damaged spillway of the Oroville Dam in Oroville, Calif.California Department of Water Resources/Getty Images

Some of this year’s precipitation fell as rain, which is a short-term challenge. But a significant portion is parked on top of the Sierra Nevada in the form of snow, and it will stay there until temperatures begin to warm in the spring. At that point, dam officials will be forced to manage two sources of inflow: melting snowpack and storm rainfall.

Alan Haynes, a hydrologist at the River Forecast Center, says they expect 2.2 million acre feet to flow into Northern California’s water infrastructure during the melt season between April and July. But there’s a considerable amount of uncertainty in that forecast this year.

“The high end is more like up around 3.4 million acre feet, if it stays wet through the spring,” Haynes told The Washington Post, “which is twice as much as average.”

The best-case scenario is an average spring runoff, Haynes added.

Winter storms and spring runoff haven’t been a problem in the Lake Oroville dam’s 50-year history. But this year is different, not necessarily in the intensity of the storms but in their persistence.

“What we’ve seen this year is moderately strong storms that come so frequently that there’s not enough time to draw down the reservoirs between them,” Haynes explained. “We can forecast about a week out with some degree of skill, but the challenge has been the water management component to balance public safety downstream with keeping the dams below capacity.”

That will be the challenge at Lake Oroville in the coming months — keeping people downstream safe while managing the dam itself. If the past is any indicator of the future, they should prepare for more precipitation, not less.